This invention relates to an integral snap hinge of plastic material.
Snap hinges are disclosed in Swiss Patent Specification No. 516,725, in which two hinge members are interconnected by a film hinge provided within the main pivoting axis. The connecting elements represent a bendable spring, which during the snap action exerts forces on the two hinge members to forcibly move them from a position of unstable equilibrium alternatively to their open or closed positions. In accordance with FIGS. 8 to 15 of the Swiss patent specification the resilient connecting elements are linked to the hinge members by two additional film hinges. The axes of all three film hinges are parallel to each other.
A known field of application of snap hinges of that kind are container closures which are integrally injection molded with the hinges. The embodiments shown in FIGS. 10 to 15 of the Swiss patent specification cannot be used for that purpose because the resilient connecting element (62 to 76) extends around the main pivoting axis (60 or 75). The provision of such snap hinge on a closure would require complicated injection molds.
However, the embodiments shown in FIGS. 8 to 9 of the Swiss patent specification can be used for closures, and they must be formed with a relatively large cutout in at least one of the hinge members for accommodating the resilient connecting elements (see numeral 42). For this reason the resilient connecting element must be separated from one hinge member and partly also from the other hinge member by slots which extend transversely to the axes.
The strength of the snap hinge is impaired by the cutout, which must have a considerable length for accommodating the resilient connecting element. The snap hinge can relatively easily be destroyed if one hinge member is twisted relative to the other about an axis which is transverse to the principal or main axis.
In a snap hinge closure for bottles or the like, the body to be fitted on the mouth of the bottle is generally closed at its one end by a bottom, which contains a discharge opening. For the accommodation of the resilient connecting element, a relatively large cutout in the cover is required as well as a cutout in the body. Without the latter cutout, the film hinges which coincides with the main axis and provide a flexible connection between the two hinge members, would protrude outwardly to a relatively large extent. This would result in an unsatisfactory appearance and would prevent the screwing or snapping of the closure onto the bottle by means of a conventional closing machine so that special machines would be required.
If part of the cutout extends into the body, the cutout will protrude somewhat into the head plate and this will restrict the latitude for the closure designer. It is not possible to make closures which have discharge openings and associated parts that are large in relation to the overall diameter.
Finally, snap hinges according to the Swiss patent specification have a technical appearance so that they often fail to meet the esthetic requirements to be fulfilled by packages.
For the following reasons, the known snap hinges cannot be used as integrated hinges of vacuum formed packages. The cutout would impair the strength of the package. The punched out waste would give rise to considerable disturbances in production. In some uses the cutout would be disturbing because it constitutes a hole in the package.